This invention pertains to a flywheel energy storage system and more particularly to an energy extraction method and circuit that provides increased energy delivery capacity for a flywheel system by having a higher output efficiency and simultaneously has a longer life and higher reliability.
Flywheel uninterruptible power supplies have emerged as an alternative to electrochemical batteries for prevention of power interruptions to critical loads. Electrochemical batteries used in these applications, in particular, valve regulated lead acid batteries, have many undesirable traits. The life of batteries is short, typically between 1 to 7 years depending on the environment and use. They require periodic maintenance and inspection are subject to thermal degradation and can fail unpredictably when required. Lead acid batteries and other types as well are also not environmentally friendly. However, lead acid batteries are relatively inexpensive. Flywheel systems show promise to eliminate the disadvantages of batteries with the expectation of achieving 20 year lives with minimal or no maintenance, temperature insensitivity, previously unachievable reliability while being environmentally benign.
A flywheel energy storage system is shown in FIG. 1. A high-speed flywheel 12 stores electrical energy in the rotating inertia of a flywheel. Flywheels can be either constructed of metal or of composite materials. The flywheel is supported for rotation using upper and lower bearings 14 and 15. The flywheel can be supported on mechanical bearings, magnetic bearings or a combination. An attached motor/generator 16 is used to accelerate and decelerate the flywheel 12 for storing or retrieving energy. Many designs of motor/generators exist and can be employed. Motor/generators can also be made as separate components. To reduce the losses from aerodynamic drag, the interior 13 of the housing 11 surrounding the flywheel 12 is maintained at a low pressure, or for slower flywheels it can be filled with a gas of small molecule size such as helium. The flywheel mechanical unit 17 is electrically connected for operation and conversion of power. Typically, utility power 21 is taken is taken in for input conversion 20 and power is supplied to a critical load 22 through output conversion 19. A system control 18 provides control for the system 10.
Regardless of the physical design employed, it is desirable for the flywheel system to both maximize its stored energy capacity and also to maximize its operating life. Such capability can offset the higher initial cost of the flywheel system over batteries by actually becoming cheaper when considered over the system life. One element of flywheel uninterruptible power supplies that deserves particular attention is the power system electronics. Designing electronics for an operating life that is preferably greater than 10-20 years without failures is challenging. Likewise, increasing the efficiency of the power system is preferable for allowing more of the stored energy of the flywheel to be delivered to the load.
One power system configuration that has been used with previous flywheel uninterruptible power supplies is shown at 30 in FIG. 2. The power system 30 takes in utility power 31 and supplies protected direct current power at the output 32. For many telecommunications systems such as telephone and wireless, the output voltage 32 required is xe2x88x9248 volts or 24 volts. For other applications, such as high power ride-through for data centers or critical manufacturing, the input and output voltages would be increased. The input power 31 is rectified to a DC bus 34 using a rectifier 33 which can be either controlled or uncontrolled. The DC bus 34 supplies power to a PWM (pulse width modulated) inverter 35 also known as a servo amplifier. The servo amplifier 35 converts the DC current in the bus 34 to synchronous alternating current 36 that provides power to accelerate the flywheel 37 to normal operating speed. When the utility power is operating normally, the DC voltage in the bus 34 is converted to the output voltage 32 using a DC-DC converter 38. During an interruption in the utility power 31, energy from the rotating flywheel supplies power to the output 32 by providing power to the DC bus. The inverter provides power to the DC bus instantly and automatically when the utility power is discontinued by antiparallel diodes included with the H-bridge, not shown, inside the inverter. Power automatically flows back and is rectified to the DC bus 34 whenever the generator voltage is greater than the DC bus. As the flywheel speed slows, the voltage to the DC bus 34 drops. The output DC-DC converter 38 maintains the constant output voltage 32 during discharging of the flywheel. The output power can alternatively be alternating current power simply by replacing the output DC-DC converter 38 with an output DC-AC converter or inverter.
Current designs of DC-DC converters typically have efficiencies of between 75-90%. The less than perfect efficiency means that all of the energy stored in the flywheel cannot be effectively used. Such levels of efficiency are acceptable for many applications, however the attention given by customers and potential customers to the cost of energy storage capacity of energy flywheel uninterruptible power supplies, achieving higher efficiency is desirable. The mean time between failure for many converters is only about 12 years, which means a significant portion of flywheel systems will fail before the end of desired operating life.
A second configuration of power system for a flywheel uninterruptible power supply, shown at 40 in FIG. 3, is similar to that shown in FIG. 2, supplying protected DC output power 42 from utility power 41. The input power is rectified to a lower voltage DC bus 44 using a switched mode rectifier 43. The output power 42 is supplied directly from the DC bus 44. The DC bus is also connected to a PWM inverter 45 that generates synchronous AC to power lines 46 to accelerate the flywheel motor/generator 47 to fully charged operating speed. During an interruption of primary power 41, the servo amplifier 45 supplies power to the DC bus 44 and output 42. The output is maintained at a constant voltage during the slowing of the flywheel by using fourth quadrant regenerative operation of the servo amplifier 45. Fourth quadrant operation uses switching with the internal inductance and capacitance to actively decelerate the flywheel. By actively decelerating the flywheel, the DC bus voltage is boosted to a higher voltage than the generator voltage. The output voltage is thereby maintained constant without the use of an output converter. Unfortunately, as the flywheel slows, the generator voltage drops and the generator current can become excessive if discharged to low speeds as desired for extraction of most of the energy stored in the flywheel. The servo amplifier 45 also operates similarly, having comparable efficiency as a DC-DC converter.
Another method used in previous flywheel systems to maintain a constant output voltage as the flywheel speed is decreased is to use a motor/generator with an external field coil. The field coil is used to create all or some of the magnetic flux of the generator. As the speed decreases, the power to the field coil is increased such that more flux is created, increasing the generator voltage. Some designs use both permanent magnets on the rotor in conjunction with the flux from the field coil. In either case, the requirement of power to generate all of the generator flux or to generate a significant portion of it as would be required for extraction of most of the energy in the flywheel is less efficient than a permanent magnet motor/generator that has a high magnetic flux without the use of any power. Field control designs also have increased areas for hysteresis and eddy current losses due to moving steel portions. Likewise, field control motor/generators can necessitate smaller operating magnetic air gaps to generate high fields, thereby making the use of magnetic bearings to support the flywheel more difficult.
The invention is a flywheel uninterruptible power supply having a power system and output regulation method that provides increased energy delivery capacity for the flywheel system by achieving a higher efficiency in converting the energy stored in the flywheel to power delivered to the load. The power system works by directly converting the variable frequency and voltage alternating current produced from the generator into the supplied output power. The conversion uses switching to regulate the generator power and the switching occurs by natural commutation such that the frequency is linked to the speed of the flywheel generator. As the generator produces alternating current voltage, the output regulator switches the generator power to the load during the half cycles. The regulator can provide portions of each half cycle to the load as in phase angle switching or in another embodiment, zero cross switching is used so that complete half cycles are provided. The power pulses from the generator can be combined from multiple phases and the voltage is then filtered to provide a constant output voltage. By switching at the low generator frequencies, typically between 300 Hz to 1 kHz from many flywheel systems, the efficiency of the power conversion is increased. Switching losses in power conversion are linearly proportional to the switching frequency and thus a significant improvement can be made over conventional conversion at frequencies of up to 20 kHz and higher. The drawback of the lower frequency switching is increased size of the inductor and capacitor filter components, however this is less important than the increased energy delivery for the applications of flywheel systems. The larger filter components can also be included inside the already large and heavy flywheel unit. Moreover in telecommunications applications, the downstream equipment usually have embedded converters so power regulation is not as critical.
Despite the efficiency increase solely from lower frequency, the efficiency of output power regulator is also increased through the switching process. Switching losses arise from the power dissipation in transition from on to off state or off to on state of the switching devices. Accordingly with the invention, the transition from on to off state occurs when the current through the switch is effectively zero, thereby effectively eliminating this loss. This is possible because the generator voltage is an alternating current which periodically passes through zero voltage. By switching off at zero current, energy in load or filter inductance is also not dissipated. The efficiency can be even further increased by also reducing the switching loss from transition from off to on by using zero cross switching where the on switching occurs at zero voltage. The regulation in this case occurs by allowing complete half cycles of the generator voltage to be switched to the load however only some half cycles are switched to the load so as to provide output voltage regulation. In one embodiment, the output voltage can also be an alternating current and the switching regulation converts the variable frequency generator voltage in to a fixed lower frequency output voltage. This is done directly without incurring losses from having an intermediate conversion to direct current power. In order for the output regulation of the invention to function and provide a constant output voltage as the flywheel speed slows, the flywheel must be accelerated to a normal operating speed in which the generator voltage is higher than the desired output voltage. It is preferable that the maximum generator voltage at full speed is more than twice and more preferably more than three times the output voltage for extraction of most of the flywheels energy. The higher generator voltage also further reduces losses by reducing resistive heating loss.
Another important benefit of the invention is the increased life and reliability of the power system. Because of the direct regulation of the output voltage from the generator, fewer components are required, increasing reliability. The lower switching frequency and the lower electronics stresses from the zero current on to off transitions help the output regulator achieve significantly higher reliability and longer life. The life can be further increased by the power system only supplying power from the flywheel regulator during infrequent utility interruptions and normal output power is supplied directly from separately converted utility power. The invention is applicable for use in both long term low power discharge flywheel systems as well as high power ride-through systems.